1. Field of the Invention
The present invention relates to a circuit and method for driving a laser diode.
One of apparatuses making up an optical transmission network is an optical transmission apparatus. This optical transmission apparatus is provided in an output stage thereof with an electro-optic signal converter which receives as its input an internally processed electric signal and converts this to an optical signal. As the light source of this electro-optic signal converter, usually a laser diode is used. The present invention will describe a circuit for driving this laser diode.
2. Description of the Related Art
As will be explained in detail later referring to the drawings, a threshold current (Ith) in a conventional laser diode is relatively large. Namely, the laser diode causes laser oscillation in a laser current region where the current exceeds a large threshold current value.
When handling increasingly high speed signal inputs in the future, however, if the threshold current is large, a light emission delay, explained later, would cause a serious problem. For this reason, attempts have been made to further reduce the threshold current and, in fact, laser diodes remarkably reduced in the threshold current have been commercialized. Such a laser diode exhibits a characteristic curve &lt;1&gt; which is plotted nearer the zero point than another characteristic curve &lt;2&gt; as will be explained later by referring to FIG. 11. In case of the curve &lt;1&gt;, the threshold current is Ith1 which is smaller than Ith2. With such a characteristic curve &lt;1&gt;, the bias current to be fed by a bias current source also becomes small, as shown by Ib1 smaller than Ib2 in the figure.
However, if the bias current becomes small as the above Ib1, when taking into account the unavoidable variations and fluctuations in the bias current sources themselves, it becomes very difficult to hold a very small predetermined bias current Ib1 with a high precision. In addition, the characteristics of a laser diode per se vary due to temperature fluctuations or aging, so there arises a problem that deterioration of the extinction ratio is almost inevitable.